This invention relates to the processing of digital data for transmission over a communications channel of limited bandwidth and power, and relates to a method and apparatus for increasing the bandwidth efficiency and the power efficiency of the transmission.
One way to increase the bandwidth efficiency of a linear modulation scheme is to increase the number M of possible signaling levels. Such an increase is achieved however at the expenses of a smaller minimum distance d.sub.min between the multilevels, for a fixed average signal power.
Csajka et al (U.S. Pat. No. 4,077,021) describe Trellis Coded Modulation (TCM) which combines convolutional coding with linear multilevel modulation schemes in an efficient way. TCM preserves the Bandwidth (BW) efficiency of the modulation schemes while offering 3 to 6 db increase in d.sub.min.sup.2 over multilevel modulation, and is considered the state of the art in the domain of power and Bandwidth (BW) efficient modulation schemes.
The present invention involves the use of a conversion filter and a bandpass filter (BPF) together with an encoder and a modulator as part of a digital signaling system. The difference between the signaling system involved in the present invention and the signaling system comprising TCM is the conversion filter which allows the signaling system to generate schemes that are more BW-efficient than the schemes generated by the encoder-modulator combination. It can be shown that the digital signaling system described in the present invention can generate schemes that have larger d.sub.min.sup.2 /E.sub.b than TCM for comparable BW-efficiency and comparable complexity of the receiver, E.sub.b being the average energy per transmitted bit.
Conversion filtering was first introduced in 1969 by Lender (Canadian Patent No. 823,307,2) in order to generate a signal of near constant envelope. In Lender, the conversion filter is an Infinite Impulse Response (IIR) filter, constrained to have a specific Power Spectral Density (PSD) and a modulation index h equal to 1/2. Subsequently, conversion filtering was used to generate signals of constant envelope (A. R. Hambley and O. Tanaka, "Generalized Serial MSK Modulation," IEEE Trans. on Commun., Vol. COM-32, pp. 305-308, March 1984 and F. Amoroso and J. A. Kivett, "Simplified MSK Signaling Technique," IEEE Trans. Commun., Vol. COM-25, pp. 433-441, April 1977). In both cases, the conversion filter is a Finite Impulse Response (FIR) filter, constrained to have a length equal to one symbol duration, a modulation index h equal to 1/2 and whose Lowpass (LP) equivalent Impulse Response (IR) g(t) is restricted to take the format ##EQU1## where .phi.(t) is a slowly varying function and j=.sqroot.-1.